text: Two helium atoms can fit inside larger fullerenes, including C and C. These can be detected by the nuclear magnetic resonance of He having a small shift, and by mass spectrometry. C with enclosed helium can contain 20% He@C, whereas C has 10% and C has 8%. The larger cavities are more likely to hold more atoms. Even when the two helium atoms are placed closely to each other in a small cage, there is no chemical bond between them. The presence of two He atoms in a C fullerene cage is only predicted to have a small effect on the reactivity of the fullerene. The effect is to have electrons withdrawn from the endohedral helium atoms, giving them a slight positive partial charge to produce He, which have a stronger bond than uncharged helium atoms. However, by the Löwdin definition there is a bond present.
  The two helium atoms inside the C cage are separated by 1.979 Å and the distance from a helium atom to the carbon cage is 2.507 Å. The charge transfer gives 0.011 electron charge units to each helium atom. There should be at least 10 vibrational levels for the He-He pair.